Interaction of the C-terminal immunoglobulin-like domains (Ig 22-24) of filamin C with human small heat shock proteins

Small heat shock proteins are the well-known regulators of the cytoskeleton integrity, yet their complexes with actin-binding proteins are underexplored. Filamin C, a dimeric 560 kDa protein, abundant in cardiac and skeletal muscles, crosslinks actin filaments and contributes to Z-disc formation and membrane-cytoskeleton attachment. Here, we analyzed the interaction of a human filamin C fragment containing immunoglobulin-like domains 22-24 (FLNC_22-24) with five small heat shock proteins (HspB1, HspB5, HspB6, HspB7, HspB8) and their α-crystallin domains. On size-exclusion chromatography, only HspB7 or its α-crystallin domain formed complexes with FLNC_22-24. Despite similar isoelectric points of the small heat shock proteins analyzed, only HspB7 and its α-crystallin domain interacted with FLNC_22-24 on native gel electrophoresis. Crosslinking with glutaraldehyde confirmed the formation of complexes between HspB7 (or its α-crystallin domain) and the filamin С fragment, inhibiting intersubunit FLNC crosslinking. These data are consistent with the structure modeling using Alphafold. Thus, the C-terminal fragment (immunoglobulin-like domains 22-24) of filamin C contains the site for HspB7 (or its α-crystallin domain) interaction, which competes with FLNC_22-24 dimerization and its probable interaction with different target proteins.